External counterpulsation provides tangible curative effect in the treatment of cardiovascular diseases, which have become more and more prevalent in recent years. In American Cardiovascular Journal (30(10)656-661, 1973) Dr. Cohen reported a device for external counterpulsation, being a four-limb sequential counterpulsation device. It consists of multiple balloons wrapped around the four limbs of the patient. Pressure is applied sequentially from the distal to the proximal portion of each limb. Using high pressure gas from a large compressor as its energy source (1000 to 1750 mm Hg) to control the opening time of a solenoid valve, the balloons receive pressurized air during inflation. The balloons are deflated by use of a vacuum pump. The device requires the use of a large air compressor, a large vacuum pump and the use of numerous pressure transducers to monitor the input pressure to insure that no excessive pressure is exerted in the balloons. However, this device is not only bulky and expensive, but it is also extremely noisy and complicated to operate. It is, therefore, unsuitable for everyday clinical use.
External cardiac assistance has been described in U.S. Pat. No. 3,866,604, which is an improvement on the above original external counterpulsation device. However, this device is extremely bulky noisy, and complicated to operated.
An external counterpulsation apparatus has also been described in Chinese patent CN85200905, which has also been granted as U.S. Pat. No. 4,753,226. This external counterpulsation apparatus is regarded as another improvement over previous art. In addition to balloons for the four limbs, it also comprises a pair of buttock balloons. The balloons are sequentially inflated with positive pressure and then, with appropriate delay, simultaneously deflated using a microcomputer to control the opening and closing of solenoid valves. The high pressure gas source and vacuum pump have been eliminated, so as to reduce the volume of the apparatus and make it more practical. However, the deflation of the balloons of this apparatus lacks the suction of negative pressure and depends on natural exhaustion into the atmosphere. Therefore the exhaustion of the balloons is incomplete and slow, and leaves behind residual gas in the balloon which hinders the ability of this device to reduce afterload (workload) of the heart.
A positive and negative enhanced type external counterpulsation apparatus has been described in Chinese patent CN88203328, wherein a negative pressure suction means for exhaustion of the balloons has been added. However, this apparatus is still ineffective in the exhaustion of all the pressurized gas in the balloons, and in addition, it is still too large, noisy and heavy for transport to be of practical application in the clinical setting.
A miniaturized external counterpulsation apparatus has been described in Chinese Patent CN1057189A, wherein the air compressor can be placed inside the main body of the device and does not require a separate embodiment. The box containing the solenoid valves and the balloon cuffs are suspended in a tube like aparatus and directly attached to the main body of the device. This device is practical for clinical use in that its size is very much reduced. However, this device does not have negative suction to increase the rate of deflation of the balloons, and it is still extremely noisy and not very efficient in producing desirable counterpulsation hemodynamic effects, namely, a high rate of inflation and effective deflation.
The foregoing external counterpulsation apparatuses have many advantages over the original one, but there are still many problems. For example, the high pressure air produced by the air compressor has a high temperature when it arrives at the balloons, which may cause discomfort feeling or even pain for the patient; the balloon cuff used by the prior art external counterpulsation apparatus is made of soft materials such as leatherette, canvas and the like, which may have a high elasticity and extensibility, requiring the use of a large volume of gas to achieve the required pressure and resulting in the inability to quickly inflate the balloons for optimal rate of inflation. Furthermore, dead space may be formed due to the misfit between the balloon cuff and the surrounded limb; the balloon cuff could slip downward during counterpulsing thereby being incapable of efficiently driving blood from peripheral regions to the root of the aorta, which directly affects the effectiveness of the counterpulsation treatment. All these factors reduce the efficiency of counterpulsation and require more pressurized gas to fillup dead space and more power from the compressor. At the same time a reduction in the rate of inflation of the balloon results which hinders the effective compression of the body mass as well as vasculature.
Historically, the earlobe pulse wave, finger pulse or temporal pulse wave is used as a timing signal to give the appropriate time for application of the external pressure so that the resulting pulse produced by external pressure in the artery can arrive at the root of the aorta just at the closure of the aortic valve. Thus, the arterial pulse wave is divided into a systolic period and a diastolic period. However, earlobe pulse wave, finger pulse wave or temporal pulse wave are signals derived from microcirculation and may not reflect the true pulse wave from the great arteries such as the arota. Using the dicrotic notch as the true aortic valve closure is incorrect because the dicrotic notch is affected by many other factors such as the dampening effect of the vascular elasticity, reflective wave from tapering of the arteries and interference from previous pulse waves. Therefore it is most important in the art of external counterpulsation to find the true aortic valve closure time so the appropriate inflation time can be found for the externally applied pressure.
Theoretically, there are two factors that should be taken into account to determine the appropriate deflation time of all the balloons simultaneously. (1) release of all external pressure before the next systole to produce maximal systolic unloading, that is the maximum reduction of systolic pressure; (2) maintainence of the inflation as long as possible to fully utilize the whole period of diastole so as to produce the longest possible diastolic augmentation, that is the increase of diastolic pressure due to externally applied pressure. Therefore one measurement of effective counterpulsation is the ability to minimize systolic pressure, and at the same time maximize the ratio of the area under the diastolic wave form to that of the area under the systolic wave form. This consideration can be used to provide a guiding rule for determination of optimal deflation time.
Furthermore, the various existing external counterpulsation apparatuses only measure the electrocardiographic signals of the patient to guard against arrhythmia. Since counterpulsation applies pressure on the limbs during diastole, which increases the arterial pressure in diastole and makes it higher than the systolic pressure, the blood flow dynamics and physiological parameters of the human body may vary significantly. Some of these variations maybe advantageous, while some of them are potentially unsafe. For patients with arteriosclerosis and phlebosclerosis, there is the danger of blood vessels breaking due to the increase in their internal pressure. Furthermore, applying pressure to the limbs presses not only on the arteries but also the veins, and this may result in an increase in the amount of blood returning to the heart. This may cause cardiac lung or pulmonary edema because of the degration of the decrease in pumping capacity of the heart and incapability of the heart to pump out the increased amount of blood returning to the heart. This may, in turn, affect the oxygen saturation in the arteries of the body and cause an oxygen debt. It is therefore necessary to monitor the maximum value of the arterial pressure and oxygen saturation in the blood of a patient in addition to monitoring the electro-cardiogram, to ensure safety of the patient during the counterpulsation treatment.
Furthermore, the gas distribution device in the existing external counterpulsation apparatuses operate by controlling the opening and closing of the solenoid valves, which has the disadvantage of having voluminous and complex pipe connections. This is disadvantageous to miniaturizing the whole apparatus and improving its portability.